1. Field of the Invention
The present invention relates to a diagnostic ultrasound apparatus capable of providing a three-dimensional image by scanning in real time a three-dimensional region of an object to be diagnosed and a method of switching over the images, and in particular, the apparatus and the method directed to a contrast echo technique with a contrast agent of which essential constituent is microbubbles, in which the contrast echo technique is suitable for observation of dynamics of flows of blood through vessels, observation of dynamics of flows of blood in an organic tissue by means of detecting perfusion, and quantitative measurement of those dynamics.
2. Description of the Prior Art
Ultrasound signals have now been clinically used in various fields, in which one usage is an application to a diagnostic ultrasound apparatus. The diagnostic ultrasound apparatus acquires image signals through transmission and reception of an ultrasound signal toward and from an object and is used in a variety of modes utilizing non-invasiveness of the signal. One typical type of diagnostic ultrasound apparatus produces tomographic images of a soft tissue of a living body by using a method of ultrasound pulse reflection imaging. This imaging method non-invasively produces tomographic images of the tissue. Compared with other medical modalities such as a diagnostic X-ray apparatus, X-ray CT scanner, MRI system, and diagnostic nuclear medicine system, this imaging method is advantageous in many aspects such that real-time display is possible, a compact apparatus is manufactured at relatively lower costs, the exposure of X-rays will not occur, and blood imaging is possible thanks to ultrasound Doppler imaging.
Thus, this imaging is widely used in diagnosis of the heart, abdomen, mammary gland, urinary organs, and obstetrics and gynecology, and possesses various advantages. In particular, pulsation of the heart or motion of a fetus can be observed in real time just through a manipulation that is as simple as placing an ultrasound probe on the patient""s surface. Still, since there is no need to worry about patient""s exposure, screening can be carried out repeatedly many times. Furthermore, there is an advantage that an apparatus can be moved to a bedside position to examine a patient easily.
In the field of this diagnostic ultrasound apparatus, for screening the heart or abdominal organs, contrast echo imaging has newly been introduced and spotlighted recently, by which an ultrasound contrast agent (hereinafter called contrast agent) is trans-venous injected into a patient for evaluating the kinetics of blood flow. Since the trans-venous injection of a contrast agent is less invasive than trans-arterial injection type of contrast echo imaging, diagnosis using such trans-venous injection technique tends to become popular. A main constituent of the contrast agent is composed of minute bubbles (microbubbles) that act as sources of reflecting ultrasound waves. The larger the amount and concentration of an injected contrast agent is, the larger the effect of contrast imaging is. However, due to some characteristic of microbubbles of a contrast agent, a situation where radiation of ultrasounds results in a shortened duration of the contrast effect. Considering such a situation, a contrast agent having characteristics of long persistency and high durability against sound pressure has been developed in recent years.
Concurrently with such development of imaging techniques, a need for three-dimensional imaging for ultrasound diagnosis has been needed, like in the fields of the CT and MRI. In a three-dimensional volume image, information about an object in its back and forth direction is acquired in addition to information obtained from a two-dimensional tomographic image, it is possible to know more clearly shapes of tissues and dynamics of flows of blood. Thus, in conducting ultrasound diagnosis, visualizing three-dimensional images has drawn attention as a way of developing a new diagnostic field.
As one way of three-dimensional scanning, there is a technique of acquiring three-dimensional echo data as a probe of which ultrasound transducers arranged one-dimensionally is moved along the body surface of a patient. Practically, a convex probe or linearly arrayed probe for abdominal imaging is moved by hand or mechanically. Alternatively, a trans-esophagus multi-plane probe having a mechanism of rotating a sector probe is used.
However, the above one-dimensional probe requires that it take a large amount of time to acquire three-dimensional echo information itself even when any scanning method is adopted, compared to the conventional cross sectional scanning method. A fast-moving object, such as the heart, is therefore difficult to be traced with high accuracy. Even if an object does not move so fast as does the heart, distortion of images becomes too large unless a probe is located in a sufficiently secured manner.
In recent years, a diagnostic ultrasound apparatus has been researched eagerly, which comprises a probe having phased array transducers arranged two-dimensionally so as to scan an ultrasound beam three-dimensionally is used to scan a three-dimensional volume region at a frame rate closer to realtime scanning, for example, 30 frames/sec.
Though well known, the advantages of a three-dimensional volume image include, obtaining information about an image in its the back and forward direction, which has not been given by the conventional two-dimensional tomographic image; making it possible to observe a region of interest from a point viewing along in an arbitrary direction; and others. Such a three-dimensional volume image (three-dimensional information) is displayed on a two-dimensional planar display, except display that uses particular volume image display devices. This display is realized by cutting out a two-dimensional section from a three-dimensional volume image so as to provide the section image, or producing a projection image viewing a three-dimensional volume image from a specified point of view so as to provide the projection image. This way to produce projection images include a maximum intensity (luminance) projection technique (Max IP technique) and a minimum intensity (luminance) projection technique (Min IP technique).
Although the techniques of visualizing the foregoing projection images are relatively easily available for displaying an object in the air, there are some cases where they cannot offer an effective display for ultrasound images.
In other words, owing to the fact that an ultrasound image into which a contrast agent has not been injected represents an object""s tissue region corresponding to the foregoing air and producing an echo signal of which intensity is relatively high, displaying a two-dimensional projected image using, for example, the maximum intensity projection technique may result in that constituents, such as tumors in tissue and blood vessels, are hidden by a tissue image portion. In this case, the minimum intensity projection technique can be used to extract blood vessels on the projection image, because echo signals from the blood vessel portion represent smaller intensities and their display luminance degrees are lower.
In contrast, when carrying out a contrast echo technique with a contrast agent injected, tissue and blood vessel systems are subject to a contrast effect and higher in luminance than organic tissue. As a result, for the contrast echo technique carried out after the start of an injection of the contrast agent, the maximum intensity projection technique is more advantageous.
In general, in cases the contrast echo technique is performed, it is not necessarily true that injecting a contrast agent is merely followed by scanning. It is necessary that both images acquired before and after injecting the contrast agent be traced sequentially in time. In the case that the maximum intensity projection (Max IP) technique or the minimum intensity projection (Min IP) technique are selectively employed, it is impossible to acquire blood vessel information endurable to those traces carried out sequentially in time. For example, when the maximum intensity projection technique is adopted, there frequently occurs that blood vessel regions are varied in an organic tissue on an image obtained before injecting the contrast agent. By contrast, if the minimum intensity projection technique is adopted, it cannot be realized that blood vessel regions are extracted securely on an image obtained after the start of an injection of the contrast agent.
The present invention has been made with consideration of the drawbacks of the above prior art. An object of the present invention is to provide a diagnostic ultrasound apparatus and a method for displaying ultrasound images, which are capable of visualizing the images in which a blood vessel is always securely depicted before and after the start of injecting a contrast agent in cases where contrast echo imaging on three-dimensional scanning is performed with the contrast agent injected.
In order to realize the above object, the present invention pay attention to the fact that, in most cases, the largeness relationship of intensity of echo signals emanated from both organic parenchyma and blood vessels is reversed when contrast echo imaging is performed on three-dimensional scanning, and has a primary feature of switching over projection techniques (that is, display techniques) between periods provided before and after the start of an injection of a contrast agent in projecting a three-dimensional volume data to a two-dimensional image. The present invention also has another feature of how such switchover timing is detected. Configurations to realize those features are detailed as follows.
A diagnostic ultrasound apparatus according to the present invention, as its basic constitution, comprises signal acquiring means for acquiring an echo signal by scanning a three-dimensional region of an object with an ultrasound beam; data producing means for producing three-dimensional image data based on the echo signal; displaying means for displaying the three-dimensional image data; specifying means for specifying a timing at which injecting an ultrasound contrast agent into the object is started; and display switchover means for switching over at the specified timing a display state of the three-dimensional image data displayed by the displaying means.
By way of example, the specifying means is composed of means for specifying, as the injection timing, an arbitrary timing specified by an operator.
Still by way of example, the specifying means is composed of means for automatically specifying the injection timing as the ultrasound contrast agent is injected. In this case, as one preferred embodiment, the specifying means has means for automatically specifying the injection timing responsively to a signal associated with a contrast agent injecting operation, the signal being given by a contrast agent injection apparatus. As another example, the specifying means has setting means for setting the injection timing from the echo signal itself. Specifically, the displaying means is means for displaying projection data obtained by projecting two-dimensionally the three-dimensional image data with a desired projection technique and the display switchover means is means for switching over the projection technique at the projection timing specified by the specifying means. A more specific example is that the setting means has at least either one of a first comparing means for specifying the injection timing by making a comparison in strength between the echo signal emanated from a tissue of the object and the echo signal emanated from a blood flow thereof and a second comparing means for making a comparison in strength between echo signals emanated from a blood flow of the object at sequential earlier and later timings so as to specify the injection timing.
In this constitution, for example, the first and second comparing means are composed of means for performing the comparison using a sum or average of intensity of at least part of the echo signal providing three-dimensional information. Alternatively, the first and second comparing means may be composed of means for performing the comparison using a sum or average of signal intensities of a group of signals constituting at least part of a region made up of the echo signal acquired at every spatial position in the three-dimensional region. Further, this comparison can be made using a sum or average of signal intensities during a certain interval.
On one hand, in the foregoing basic constitution, it is preferred that the display switchover means is composed of means for switching over an image display state on a first display technique used before injecting the ultrasound contrast agent to an image display state on a second display technique used after start of injecting the ultrasound contrast agent. In particular, it is preferred that the first display technique is composed of an image display technique displaying projection data obtained by projecting two- dimensionally the image data using a minimum intensity projection technique; displaying data produced by reversing in luminance gradations projection data obtained by projecting two-dimensionally the image data using a minimum intensity projection technique; or displaying projection data obtained by projecting two-dimensionally image data using a maximum intensity projection technique, the image data being produced by reversing the produced image data in luminance gradations. In contrast, preferably, the second display technique is composed of an image display technique displaying projection data obtained by projecting two-dimensionally the image data using a maximum intensity projection technique.
Furthermore, in the diagnostic ultrasound apparatus including the basic constituent, the data producing means may comprise memory means for recording the three-dimensional image data produced in response to the scanning, replay commanding means for making the displaying means display the three-dimensional image data recorded in the memory means, and commanding means capable of commanding, during a replay of the replay, a switchover of a display state of the three-dimensional image data from an image display state on a first display technique suitable for imaging of a blood vessel before injecting the contrast agent to an image display state on a second display state suitable for imaging a blood vessel after start of injecting the contrast agent. This makes it possible to keep on observing images on which blood vessels are distinctly shown all through the periods provided before and after injecting a contrast agent even when images are replayed.
In each foregoing constitution, a harmonic echo technique can be practiced together. To realize this, the signal acquiring means comprises means for passing an only certain frequency component of the echo signal.
Incidentally, the diagnostic ultrasound apparatus according to the foregoing basic constitution may comprise an acquisition unit to acquire an echo signal by scanning a three-dimensional region of an object with an ultrasound beam; a data production unit to produce three-dimensional image data based on the echo signal; a display unit to display the three-dimensional image data; a specifying unit to specify a timing when an ultrasound contrast agent is started to be injected into the object; and a switchover unit to switch over at the specified injection timing a display state of the three-dimensional image data to be displayed by the display unit.
Alternatively as another aspect of the diagnostic ultrasound apparatus according to the present invention, in performing a contrast echo technique with an ultrasound contrast agent injected into the object, the apparatus may comprise switchover controlling means for switching over a display state of the three-dimensional image data displayed by the displaying means between a first display technique used for displaying information about a blood vessel when the echo signal emanated from a tissue of the object is larger in intensity than the echo signal emanated from blood through the blood vessel of the object, and a second display technique used for displaying information about a blood vessel when the echo signal emanated from blood of the blood vessel of the object is larger in intensity than the echo signal emanated from a tissue of the object.
On one hand, in order to accomplish the foregoing object, a method of switching over display of ultrasound images, the comprising the steps of: acquiring an echo signal by scanning a three-dimensional region of an object with an ultrasound beam not only producing three-dimensional image data based on the echo signal but also specifying a timing at which an ultrasound contrast agent is started to be injected into the object; and switching over display states of the three-dimensional image data displayed by display means at a specified timing. Specifically, by way of example, the display states are switched over from a state of projection data of the three-dimensional image data based on a minimum intensity projection technique performed before injecting the ultrasound contrast agent to a state of projection data of the three-dimensional image data based on a maximum intensity projection technique performed after start of injecting the ultrasound contrast agent.